Carbon and metal oxides on which metal nanoparticles are loaded have been studied as catalysts and adsorbents for various organic and electrochemical reactions, including epoxy reaction, Heck reaction, oxidation and reduction in a fuel cell or a lithium-ion battery, hydrogen storage, or the like. Especially, a fuel cell which converts chemical energy to electrical energy via an electrochemical reaction by supplying fuel such as hydrogen or methanol is a next-generation, environment-friendly energy source, requiring no exchange or recharging of the cell as long as fuel continues to be provided to the cell. The fuel cell has been gaining attentions as an alternative energy source for environment-friendly automobiles, home and portable electronics and is being studied actively. The electrode performance of a fuel cell is greatly affected by the chemical composition, size, distribution, stability, etc. of catalyst nanoparticles for the oxidation and reduction electrodes. Further, it is also significantly affected by the easiness of mass transfer such as diffusion of reactants to the catalytic layer or discharge of reaction products, which in turn is related with the reaction surface area, pore structure, distribution and connectivity in the electrode catalytic layer.
Especially, proton exchange membrane (PEM) or polymer electrolyte membrane (PEM) fuel cells are of great interest as an energy source for applications requiring large power such as vehicles and transportations. The principle of the PEM fuel cell is as follows. The fuel cell includes an anode, a cathode and a polymer electrolyte membrane which physically separates them. At the anode, hydrogen or alcohol such as ethanol, methanol, etc. is supplied, and oxygen is supplied to the cathode. If a circuitry is constructed after connecting wires to the anode and the cathode by, for example, connecting an external power consuming circuit, the fuel cell begins operation. Recently, technical developments of a hydron-fed PEM fuel cell or direct-methanol fuel cell (DMFC), a type of a PEM fuel cell, have been made actively and commercialization is not distant. However, the commercialization of such fuel cells is hindered by some challenges, one of them being the high cost of platinum (Pt)-based catalysts. To solve this problem, efforts have been made to improve catalytic activity and thereby reduce the usage of platinum.
In general, it is known that the activity of a catalyst loaded on a support is largely dependent on the size and dispersity of metal particles. In order to reduce the size of platinum or platinum-M (M=Ru, Rh, Mo, Os, Ir, Re, Pd, V, W, Co, Fe, Se, Ni, bi, Sn, Cr, Cu, Ti, Au, Ce, . . . , Fe—Mo, Fe—Co, Ru—Fe, Ru—Co, Ru—Mo, . . . , Ru—Fe—Co, Ru—Fe—Mo, . . . etc) alloy particles and improve dispersity, various catalyst synthesis strategies have been developed, including impregnation reduction, colloidal methods, microemulsion methods, ion exchange, or the like. However, most of them are unsuccessful in adequately controlling particle size and dispersion, or are very complex, time-consuming and ineffective in removing residual protective agents.
Although these methods allow a relatively small and narrow size distribution of catalyst nanoparticles at low loading conditions of 20 to 30 wt %, the size, distribution and dispersion become unsatisfactory at high loading of 40 wt % or more. Further, since the synthesis is mainly carried out at high temperature, and expensive solvents and sometimes environmentally harmful chemicals are used, the methods are disadvantageous in cost and may result in environmental pollution. Also, the complex processes involved are time-consuming and, especially under metal loading, it is difficult to control the particle size, distribution and dispersion. Accordingly, there is an increasing need on the development of a stable, easy and effective catalyst synthesis method allowing the preparation of platinum or Pt-M alloy nanoparticles that can be dispersed uniformly and loaded stoichiometrically uniformly.